The present invention relates to optical waveguides, and more particularly to an improved system for delivering constituents of a waveguide blank to a deposition site.
It is now well known that light, whether modulated or unmodulated, can be caused to propagate within an elongate transparent body (such as a strand of glass or the like) in discrete modes if certain preconditions are met. The size of the strand, the radial gradation in refractive index, and other considerations combine to determine the effectiveness of the strand as a transmitting medium for optical communications. In order to transmit the light without an excessive "spread" among propagation modes, or to allow only predetermined modes of light to propagate, the internal characteristics of the strand must be closely controlled.
It is also well known that a strand appropriate for use as an optical waveguide can be formed by heating a cylindrical blank of a transparent dielectric material, such as glass, and drawing the blank into the desired thin, elongate structure. As is now well known to those skilled in the art, the structural characteristics of the waveguide closely emulate that of the blank from which it is drawn, particularly the gradient of the refractive index.
More particularly, according to one popular practice a drawing blank is formed by coating a rotating, cylindrical starting member with successive layers of a sinterable glass soot. The soot is built up, layer by layer, from minute siliceous particles which are applied to the surface of the blank through the mechanism of an oxidizing reaction flame or the like. As will be recognized by those skilled in the art, the flame used for sintering and transferring particles to the surface of a glass blank or the like has in the past been termed a "hydrolyzing flame". Although the precise phenomena involved are not yet fully understood, recent studies suggest that the actual reaction is more properly characterized as oxidation. Accordingly, the description of this reaction with respect to a presently preferred embodiment will use the term "oxidizing", it being recognized that the precise nature of the chemical reaction involved is not a material factor in practicing the present invention. A siliceous matrix material, such as silicon tetrachloride, is supplied in the form of a vapor to a burner from which the flame issues. Other materials, herein termed dopants, are also supplied in controlled amounts and at particular times to vary the optical characteristics of the end product.
The various vaporized or nebulized materials then combine with oxygen in the burner flame to form tiny spherical particles, which are maintained in the molten state and propelled toward the surface of the blank by the force of the flame. In this manner the deposited material, commonly termed "soot", is laid down along a spiral locus, layer by layer, the various layers merging together to form a continuum.
One process of the type described is disclosed in U.S. Pat. No. 2,272,342-Hyde and 2,326,059-Nordberg. In order to effect a radial variation in the index of refraction of the blank material, thereby to provide a concentric "cladding" member lying outwardly of a center (core) portion, the composition of the soot is changed at a predetermined time. The dopants may be increased, discontinued, or otherwise changed so that the radially outer portion or cladding of the blank exhibits a lower index of refraction than the inner portion thereof. The interface between the differing compositions then serves to define the boundary of a waveguide core within which optical signals may propagate. Explanation of this phenomenon and other pertinent information may be found in "Fiber Optics Principles and Applications" by N. S. Kapany, Academic Press (1967).
In order to deliver the siliceous matrix material and dopants to the region of an oxidizing reaction flame the materials are prepared in liquid form, then vaporized and conventionally transported to the region of the flame by carrier gas such as oxygen. One example of prior art apparatus for carrying out this procedure is disclosed in U.S. Pat. No. 3,826,560-Schultz. The constituents from which the blank is to be formed, such as silicon and a dopant such as germanium, are provided in the form of a liquid, ordinarily silicon tetrachloride and germanium tetrachloride. The liquid materials are confined in closed reservoirs and a carrier gas such as oxygen is introduced into the reservoirs beneath the level of the liquid. The carrier gas then bubbles upwardly through each liquid, entrainting vapors of the liquid and the resultant vapor is drawn from the reservoir and transported to the site of the oxidizing reaction flame. The silicon and the dopant there combine with free oxygen in the flame to form silicon oxide and dopant oxides, which materials are deposited upon a waveguide blank or other substrate.
While the foregoing system ordinarily performs adequately and is relatively straightforward in its basics, in fact highly accurate control mechanisms are required to achieve satisfactory operation. For prior art systems of the type described several highly accurate control loops must be provided to assure the proper flow of each component material. In particular, the mass rates of flow of the carrier gas through the liquids must be constantly monitored and closely coordinated, and moreover the relative delivery rates of the matrix material and the dopant materials must be closely related. This in turn requires close control of liquid temperatures, levels and pressures. Finally, in some cases difficulty has been encountered in providing sufficiently dense vapors to the region of the oxidizing reaction burner.
From the foregoing, it should now be understood that it would be highly desirable to provide a system for delivering waveguide materials to a reaction burner or the like which obviates the above disadvantages, and provides a controllable flow of highly densified vapors of the desired materials.
It is therefore an object of the present invention to provide improved apparatus for delivering waveguide constituents to a reaction burner or the like.
It is another object of the invention to furnish a system for delivering waveguide materials which requires a less intricate control system than prior art approaches.
A further object of the invention is to provide a system for delivering waveguide constituents from their respective reservoirs without the need for a carrier gas.
Still another object is to furnish simplified apparatus for metering waveguide component materials.